Accurate position detection in wireless power transfer using magnetoresistive sensors for implant applications

Abstract

Summary form only given. Recently, the utilization of magnetic resonant coupling (MRC) mechanism for wireless power transfer (WPT) has been actively investigated. Among numerous applications of WPT technology, the energization of implanted biomedical devices wirelessly and uninterruptedly from external supply is important because it can eliminate possible device replacement due to battery depletion. Given the implanted receiver is invisible from the external transmitter, the coil misalignment occurs easily which results in low transfer efficiency and high magnetic field leakage, and consequently endangers the human health [1]. Thus, the development of accurate position detection of the implanted receiver from the external transmitter is highly desirable. Presently, most studies of the position detection in WPT are focused on the application of electric vehicles. Although the corresponding technologies such as coil sets [2] or auxiliary multi-coils [3] have achieved fruitful outcome, they are too bulky and complicated to be used in implant applications. Due to the advantages of high stability, small size, low power consumption and high precision, the magnetoresistive (MR) sensor has been widely used in many industrial applications [4]. Generally, they are arranged in an array style to measure the magnetic field vector [5] or a moving magnetic object [6]. To the best of authors' knowledge, the application of MR sensors in WPT is absent in literature. In this paper, a new position detection approach in WPT is proposed and implemented, which is particularly suitable for implant applications. The key is to use a MR sensor array to directly measure the variation of magnetic field so as to precisely detect the relative position of the implanted receiver from the external transmitter. Therefore, the advantages of efficient and compact WPT for implant applications can be achieved. Fig. 1 shows the structure and equivalent circuit of the proposed WPT system using a MR sensor array. The transmitter and receiver are connected to their compensation capacitors in series, and operate at the same resonant frequency to achieve the desired MRC. And 12 single -axis MR sensors are distributed evenly along a circular array in the inner part of the transmitter. The MR sensor array is arranged along the vertical sensing direction and is energized by a low DC voltage. For simplicity, the coil misalignment is assumed to be along the horizontal direction only. Hence, the position detection can be considered as the detection of the misaligned orientation and departed displacement. The MR sensor output is amplified by the operational amplifier (OP) and then collected by the data acquisition (DAQ) card. Consequently, based on the proposed MR sensor array detection system, the external transmitter can be adjusted to achieve accurate alignment with the implanted receiver so that the transfer efficiency can be improved while the magnetic field leakage can be suppressed. Firstly, theoretical equations of the proposed system are deduced to assess the relationship between the distribution of magnetic field and the relative position of the implanted receiver. Secondly, the magnetic field distributions under different misalignments are analyzed by using finite element method based software JMAG. As shown in Fig. 2 (a), two typical cases of misalignment are analyzed: Case 1 is 30 mm misalignment along the 225 0 direction; Case 2 is 20 mm misalignment along the 90° direction. The corresponding magnetic field distributions are shown in Fig. 2 (b) and (c), respectively. Meanwhile, the magnetic fl ux densities at 12 points that can be captured by the MR sensor array are shown in Fig. 2 (d). It can be observed that the misaligned orientation is in coincidence with the location of the maximum sensor output and the departed displacement is in inverse proportion with the sensor output. Therefore, both the misaligned orientation and departed displacement can be detected by measuring the magnetic field distributions to accurately locate the position of the implanted receiver. In this paper, an accurate position detection in WPT using MR sensors has been proposed and implemented for implant applications. The crucial point is to employ a MR sensor array to detect the variation of magnetic field so that the implanted receiver position can be accurately located. Theoretical analysis, numerical simulation and experimental results are given to validate the proposed system.